Preparation, Magnetic Properties, and Pressure-Induced Transitions of Some M~(II)M~(IV)F_6(M~(II)=Ni, Pd, Cu; M~(IV)=Pd, Pt, Sn)Complex Fluorides

摘要

M~(II)M~(IV)F_6(M~(II)=Ni, Pd, Cu; M~(IV)=Pd, Pt, Sn) and PdSnF_6 complex fluorides have been synthesized via different preparative methods using either BrF_3 as oxidizer and solvent , or solid state reactions . For M~(II)=Ni, Pd, the phases crystallize in the rhom bohedral space group R3-bar (LiSbF_6 type).Cationic ordering has been studied by X-ray diffraction and ~(119)Sn Mossbauer resonance for PdSnF_6. A lowering of symmetry has been observed when the involved divalent cation presents a Jahn-Teller configuration (Cu~(II)). Except for PdSnF_6, which is paramagentic down to 4K, all compounds are Pd_2F_6-type ferromagnets at low temperature. This behaviors has been related to the ordering between half-filled e_g orbitals of the divalent cation and empty e_g orbitals of the tetravalent cation. A drastic increase in conductivity has been observed under high pressures.In particular the insulatorsemiconductor transition induced under pressure (up to 80 kbar) in Pd_2F_6 corresponds to a decrease of the electrical resistivity by six orders of magnitude .The assumption of an electronic transition induced under pressure from mixed oxidation states (M~(II)+M~(IV)) to an unique trivalent M~(III) oxidation state has been proposed